add comment why tvOS CI job is separate from the matrix #994

Job	Run time
Test aarch64-apple-tvos	2m 41s
Test (stable)	32s
Test CUDA support	32s
Clippy	9s
Test (beta)	28s
MSRV (ubuntu-latest)	18s
Test (nightly)	25s
MSRV (windows-latest)	37s
Rustfmt	3s
Test (linux32)	51s
Test (macos)	1m 30s
Test (aarch64-ios)	58s
Test (win32)	1m 37s
Test (win64)	2m 2s
Test (mingw32)	1m 23s
Test (mingw64)	1m 32s
Test (windows-2019)	2m 0s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 6s
	19m 44s

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